One carb vs duals, theoretically speaking

There is one constant with Nortons, they like and need to breath. Kinda like animals.
Here is an analogy,
Plug one of you nostrils by soaking a cotton ball in super glue and shoving it up there. Now slap some gorilla tape across your mouth and go jogging.
Report back when you get out of the ICU.
Now if you just get up walk around, slowly I might add, you may be ok.

This may be on the drastic side but it make a point, at least theoretically anyhow.

This is all very interesting, and there are people here who have way more knowledge and experience on this subject than I will ever have.

The original question was to do with which set up gives more power and why? If I have that wrong I'm sorry to powerdoc.
Assuming we are talking about what is redilly available. At least that's the way I read the original question. Sorry again if I got that wrong too.

There are too many things that influence how much fuel/air mixture can be "sucked" into a Commando cylinder.
Air cleaner, or velocity tube length, shape, still air or in the breeze,
Carb throat size,
Manifold length and shape,
Head port length and shape,
Valve size and angle,
Cam size (lift) and duration, and overlap,
Piston stroke,
Exhaust port size and length,
Exhaust valve, size, duration and lift,
Header shape and length,
Muffler size and shape,
Etc,

A Commando engine is (by comparison) a slow reving engine which has good torque and, in my opinion, isn't worth chasing high rev performance.
I enjoy my Commando for what it is.
There are people here who race them and good luck to you and I admire your skill in making them as fast as you have. I have raced and built race engines and appreciate the amount of work that goes into them,,, And MONEY.
The above air/fuel govening factors are only just the start to getting more power from any engine.

graeme

Ok, how about this one. You a 65 Chevy nova with a 350. Your choice is 2 barrel for economy or 4 barrel for performance.
Or one 4 barrel for performance or duel quads for outrageous performance.

Another one may simply be Nascar restrictor plate racing. Google it.

Sorry to simpify, where hobot when ya need a complication.

Depends if where trying to set the Land Speed Record , or the Lap Record at Riverside , as it where .Where do you want it ?

Rohan said:

It is oft quoted that the early Model 7 Dominator twin that Bert Hopwood designed circa 1948-9 initially had a Y-branched manifold (cast into the iron head no less) and this gave terrible performance when actually run on the road. The solution was to cut off the y-branch, and substitute a U-shaped bolt-on alloy manifold. Which gave better all round performance. Be interesting to see dyno figures for both, since no examples of the y-branch version seem to have survived ?? (although pictures of it appear in all the Norton brochures for the new twins for early 1949 )

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So same size single carb had more spunk until it run out of flow capacity, cool.

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Since I see my post may have been a little ambiguous and some misunderstood my jottings, perhaps I should just clarify that the early 1949 Norton Dominators ALL had single carburettors. As designed by Bert Hopwood, the earliest had a y-shaped manifold cast into the head, with single carb. Which proved terrible on the road, so much for the fancy y-shape to it. The solution was to cut off the y-shaped manifold, and substitute an inverted U-shaped manifold - still with the same single carb, which gave reasonable performance*- so the new model could now be put on sale.

*Bear in mind that the first dommie had an all iron head and in a plunger 'garden-gate' frame - weighed a ton. 440 lbs and half the horsepower of a Commando is what you tell those asking.
Performance was not scintillating, although good for the times.

In fact, dommies didn't have any real go until the twin carb models appeared in the early 1960s. rapidly followed by cylinder heads with more downdraft ports. Those slanted down carbs are the telltale mark of the early SS Dommie models...

Cheers.

GRM 450 said:

A Commando engine is (by comparison) a slow reving engine which has good torque and, in my opinion, isn't worth chasing high rev performance.

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Compared to cars of the time, a Commando IS a high revving engine !!! ?

I recall taking an 850 head to an engine shop, to get a pesky spark plug thread helicoiled.
The mechanic took one look at those long straight downdraft inlet ports -and whistled...
And "What is that off" !!

Engines that went to 7000 rpm back then you could count on the fingers of a few hands. Until ohc became commonplace, on even fewer....

And when I took my head to the premier engine specialty shop for some valve work, their head mechanic looked at the combustion chambers, smiled, and said "Hemi", and "no wonder Nortons were so revered back in the day"

Yeah, our motors can indeed deliver some bursts of high rpm fun.

If all I wanted was to putt about under 4500rpm I would not own a Norton Commando.

We are in the bible: "and the sound of Joshua's Norton was heard throughout the land"

Complications abound in hobot's head intakes. Two conflicting conditions to merge, flow resistance and flow velocity. Keep in mind the average rule of thumb, it takes up to 150 cfm to make 100 hp. Our 32 mm carbs flow about 160 cfm max on bench measures. Going with that logic it ain't the carbs restriction power as much as head and valves and exhaust and lack of hi rpm flow though.

The bigger the total opening the less flow restriction, so lots of little tubes or one big tube is better than too few little tubes or too small a signal tube, as far as total volume they can supply
yet...
Too big a tube or too many small ones and mixtures velocity goes down as well as the fuel picked up so tends towards lean-ness/less power. Too small a tube with higher velocity may carry more fuel density per rpm but is more resistance to high rpm volume flow.

Short intakes favor hi rpm flow while longer tracks the lower rpms.

So I'd invent a tiny carb like enrichers to start and idle on, then a small carb on long tube to chirp tire off idle and diddle about at slow speed, then a large carb on short intake to take it from there.

There is a way to slice though this Gordeian fueling knot I'm applying to Ms Peel but its not applicable to this discussion and is against the grain of traditional proven ways to do it.

Wasn't it one of the Yamahahaha dirt bikes (550 ?) that had twin carbs (on a single cylinder bike), one large and one small. Same idea.
After a short while of working well while new, the synching of these things became diabolical, and owners junked them and fitted one bigger carb.

Speaking of synching carbs, I see that a magazine survey some years back of Commandos said that 70% of bikes encountered had badly synched carbs (Don't know how they judged that - they were looking for this though).
After those results, fitting a single carb would give much better running, to 70% of owners ? !!

If my current setup ever falters, I may venture in this direction.

http://cgi.ebay.com/ebaymotors/Mikuni-T ... 4cfaaba972

The TM34mm flatside was the best single carb setup I had used. Much better than the VM36. The MK2 34mm was dullsville.

Although i feel there is no single carb that will run my particular engine specs, the old Mik 4omm easy kit for 883 sportsters seems awfully tempting. But then again so does a JH Mag. Big o fist sticking out the side, clean up all the crap under the tank.

I would think that something like an SU carb fitted to a properly designed manifold would work very well on a Norton twin, and while twin carbs are likely to provide a little more top end power, a good single carb set up should mean a bike is more tractable and nicer to ride.

SU work pretty good I've heard until the throttle is snapped, then its nice and easy and economical gather of speed i hear.
Now if the SU had a variable venturi throat and an accelerator pump like a Predator carb for hot cars I'd sure be tempted. I've had a hot V8 with vacum operated sencordaries, that just bogged down on throttle jabs, until I hard wired the big butter flies so I control the thrust and tire spin by my demands. Yeah I could bog it by too fast a jab but with a bit of skill its didn't delay the power come on.

Carbonfibre said:

I would think that something like an SU carb fitted to a properly designed manifold would work very well on a Norton twin,

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There was of course the Phoenix SU carb kit available for Nortons.
While they were reportedly economical and a good touring setup, performance was not a benefit that came with the kit.

I'm curious what a "well designed manifold" would look like. ?
The problem with manifolds is that , by their very presence, they interupt the straight flow of gases into the combustion chamber. Designing this out would eliminate the manifold ??

P.S. Some Triumphs (650s ?) in the early 1950s came with the SU as standard. Not quite sure when they disappeared, or what the reasons were. When performance became a selling point ?

If the brand of carb doesn't matter, Bing or Keihin make some very well tried and proven CV carbs that are way less cantankerous, and cumbersome, than the SU.
And don't leak either !!

Who into Commando's want a pussy cat economy cruiser only when ya can have both at once, which I think is the point of this subject. A single carb/throat should have straightest shots into the head so the split in path should not be right at the head as most our 1 into 2 manifolds do, with a pair of allen head bolts cluttering it up right there to boot. Here's what Tom Drouin came up with.

Did this fall from the sky in 1947 in Roswell, NM? WTF is that?

Did this fall from the sky in 1947 in Roswell, NM? WTF is that?

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LOL! that's exactly the reaction I want from the spanked superbikers! That be the basic thrust helical stabilizer vortex generator tubage, there's a bunch of other WFT's double takers on top of that to come.

After reading this thread with great interest, I am still going to settle for the twin keihins from JS, so I can remove the stock Amals give them a birthday and look all original if and when the bike becomes a 'show pony' and still keep riding. Stock Amals work fine, so will give them a break for awhile, and they will still work fine when they go back on. Will keep you all posted about the difference between the two when I can
Regards Mike

hobot said:

Did this fall from the sky in 1947 in Roswell, NM? WTF is that?

Click to expand...

LOL! that's exactly the reaction I want from the spanked superbikers! That be the basic thrust helical stabilizer vortex generator tubage, there's a bunch of other WFT's double takers on top of that to come.

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Gee Hobot, what does the round of .45 ACP do in the assembly?

Vintage Paul

Where you been, Paul. It holds the waste gate open.

Put there because bullets look cool? or maybe to show the size relationship of that assembly to a bullet?